As a measurement showing the physical properties of a fuel, the octane number is provided which is the indicator showing the anti-knock property of the fuel. The octane number of isooctane having a high anti-knock property is 100, and the octane number of normal heptane having a low anti-knock property is 0. The octane number of a fuel is the number corresponding to a volume percent of isooctane in a mixed fuel (standard-mixed fuel) containing isooctane and normal heptane, the anti-knock property of the mixed fuel being set equal to the anti-knock property of the fuel. That is, when the anti-knock property of a fuel is equal to that of a mixed fuel in which the mixture ratio of isooctane is 50% in volume percent, the octane number of the fuel is shown as 50.
There are octane numbers of various types depending on methods or the like to determine them. For example, the research octane number adopted as the indicator in Japan, and the motor octane number adopted as the indicator in Germany are provided as the major octane numbers.
When obtaining the octane number of a fuel having an unknown octane number, the octane number is determined by performing experiments based on the above-described determination methods for the octane number.
Specifically, as shown in Non-Patent Document 1, for the research octane number and the motor octane number, a CFR engine designed exclusively to measure octane number is operated using the above fuel, and the pressure fluctuation features in a combustion chamber in this operation are compared with that in a case of operating the engine using the standard-mixed fuel. Thereby, the above octane number is determined.
The pressure values in the combustion chamber of an internal-combustion engine are changed depending on occurrence or non-occurrence of knocking. Therefore, the occurrence or non-occurrence of knocking is determined by measuring the pressure values in the combustion chamber, and the measurement timings thereof and the measured values are compared between using the fuel as a determination target and using the standard-mixed fuel. Thereby, the above octane number can be determined.